In recent years, furniture such as lockers and desks have more and more come to be made of metallic materials in order to conserve wood. The metal sheet or strip used for manufacturing such furniture is required to have a high degree of flatness.
Generally, when a metal material is rolled into a strip or sheet, the rolling is done by using rolls, and during such rolling the rolls themselves are often subject to elastic deformation, or subjected to unequal distribution of the rolling force in the direction of the roll length so that the finished metal strip or sheet is not strictly uniform in thickness, and further the thinner portions of the metal sheet or strip rolled in this manner suffer from local metal imperfections such as "center-bucklings" (A, FIG. 1-a) and "wave edges" (B, FIG. 1-b). Metal sheet or strips with these defects will greatly reduce the commercial value of furniture manufactured therefrom, and it is necessary to improve flatness of the metal sheet or strip and eliminate such imperfections.